Negative ions, such as H.sup.- and D.sup.-, have many useful applications in high-energy accelerators and plasma fusion devices for diagnostics, neutral beam heating, and current drive. The most recent high energy accelerator applications have been for neutral particle beam (NPB) weapon systems where negative ions are produced, accelerated to high energies, directed toward a target, and neutralized by stripping the excess loosely bound electrons so that they propagate toward the target, through the earth's magnetic field, as a neutral beam. A very low emittance beam is required for the beam to be of a reasonable size (about the size of the target) at a distant target.
This need for low emittance, high brightness beams places severe requirements on the accelerator, magnetic optics, neutralizer, and in particular, the ion source. If the beam is not degraded and current is not lost in accelerating the beam, then the output brightness of the system will be close to that produced by the ion source. The current and emittance can never be any better than that produced by the ion source.
The normalized root-mean-square emittance (.epsilon..sub.n,rms) at the ion source is given by ##EQU1## where r is the radius of the extraction aperture, k is Boltzmann's constant, mc.sup.2 is the ion rest energy, and T.sub.i is the temperature of the negative ions just after extraction from the plasma in the ion source. Equation (1) is just an expression of the Heisenberg uncertainty principle, since (T.sub.i).sup.1/2 is proportional to the random momentum of the negative ions in the beam and r is proportional to its uncertainty in position. The brightness, B, is then given by EQU B=I/.pi..epsilon..sup.2 =4Imc.sup.2 /.pi.r.sup.2 kT.sub.i =(4mc.sup.2 /k)(J/T.sub.i) (2)
where I is the current and J is the current density.
Two types of sources are being used for NPB weapons technology development. Surface plasma sources, which produce high brightness beams but only for short pulses and low duty factors, and multicusp chamber or so-called "bucket" type volume sources that operate continuously (CW), but have yet to produce high brightness beams. It appears that neither of these sources, as now configured, will meet the brightness requirements for an advanced neutral particle beam weapon system, nor does it appear that any reasonable modification of these configurations will lead to the desired source.
To produce an ion source that meets or exceeds the required brightness levels, it will be necessary to make a somewhat radical departure from established approaches without losing sight of the lessons learned from these previous efforts. In any negative ion source, there are mechanisms or reactions that produce the negative ions and processes that destroy them. The number of negative ions per unit volume is the result of a balance between those constructive and destructive processes. At the extraction aperture, where the useful beam is generated, the extraction itself becomes one of these destructive or loss mechanisms, and the only negative ions that can be extracted are those that are formed within one mean free path of the most dominant loss mechanism. Therefore, the optimum ion source will consist of independent mechanisms for maximizing the desired conditions. The negative ion production process that yields the lowest temperature ions in a CW source must be maximized while also minimizing the conditions for the process that most quickly destroys them. These conditions need only be produced and maintained near the extraction aperture.
The lowest temperature negative ions are produced by dissociative attachment of low energy electrons to hydrogen molecules in high vibrational states, EQU e.sub.1.sup.- +H.sub.2 *.fwdarw.H.sub.2.sup.- .fwdarw.H.sup.- +H(3)
where H.sub.2 * is a hydrogen molecule that is vibrationally excited. In equilibrium, the two dominant destruction processes are collisional detachment caused by fast electrons EQU e.sub.f.sup.- +H.sup.- .fwdarw.2e.sup.- +H (4)
and associative detachment EQU H+H.sup.- .fwdarw.H+H+e.sub.1.sup.- ( 5)
However, during extraction, after the H.sup.- ions have been accelerated somewhat, collisional stripping EQU H.sup.- +H.sub.2 .fwdarw.H+H.sub.2 +e.sup.- ( 6)
can also be a significant loss mechanism and, in some sources, collisions with walls may play a part in the kinetics as is disclosed by J. R. Hiskes and A. M. Karo, "Analysis of the H.sub.2 Vibrational Distribution in a Hydrogen Discharge," Applied Physics Letters 54(6), Feb. 6, 1989, pages 508-510.
Existing multi-chamber sources produce the vibrationally excited H.sub.2 molecules in one chamber by use of a low voltage (approximately 100 volts), high current (several hundred amps) discharge. A second chamber is produced by the use of a magnetic filter that prevents the high energy electrons from entering this second chamber. Low energy electrons cross the filter field, possibly as negative ions produced in the first chamber near the filter. Another possibility is that the low energy electron travels in conjunction with a positive ion across the magnetic filter and therefore the pair appears as a neutral entity. This indicates that H.sup.+ and H.sub.2.sup.+ production may also be important to carry low energy electrons into the second chamber in these sources. After entering the second chamber, these electrons become detached from the ions and produce the low temperature plasma found there.
The H.sup.- ions formed in the second chamber are then extracted to form the negative ion beam. The discharge in the first chamber produces not only the desired vibrationally excited H.sub.2 molecules, but also many other species such as H, H.sub.2.sup.+, and H.sub.3.sup.+ that can cross the magnetic filter and contribute to the reactions that occur in the extraction chamber in an undesirable way. This limits the brightness that can be obtained from these sources as is noted by J. R. Hiskes, "Review of Progress in the Theory of Volume Production" in a paper presented in the Fourth International Symposium on the Production and Neutralization of Negative Ions and Beams, Brookhaven National Laboratory, Oct. 27-31, 1986.
The surface plasma sources also utilize a low voltage, high current arc that must be confined to a region near a cesiated converter surface. This makes these sources more complicated and even more difficult to use and understand than the multi-chamber CW sources.
The use of a low voltage, high current discharge in all these sources means that their most sensitive parameter, the ratio of the electric field (E) to the number of particles per cubic centimeter (n), cannot be optimized simultaneously for both desired discharge operation and production of H.sup.- ions. The operation of the arc or discharge is critically dependent on this ratio (E/n), and the population of the H.sub.2 molecules in the highly vibrationally excited states is even more sensitive to E/n, as evidenced by the increase in H.sup.- current with an increase in arc power. The range of values of E/n for which the arc operates satisfactorily is far from the optimum value for the excitation of H.sub.2 to high vibrational states by collisions with fast electrons. Therefore, in the conventional sources, the best operating conditions are established by a tradeoff between the two conflicting requirements on the arc power and the H.sub.2 pressure or flow rate. An ideal source would allow for these two requirements on E/n to be optimized independently.